There are vast deposits of oil shale throughout the world with some of the richest deposits being in the western United States in Colorado, Utah and Wyoming. These reserves are regarded as one of the largest untapped energy reserves available. The oil shale is in the form of solid rock with a solid carbonaceous material known as kerogen intimately distributed therethrough. The kerogen can be decomposed to a synthetic crude petroleum by subjecting it to elevated temperatures in the order of about 700.degree. to 1500.degree. F. This causes the kerogen to decompose to a hydrocarbon liquid, small amounts of hydrocarbon gas, and some residual carbon that remains in the spent shale. Such decomposition by heating in a retort, which can be formed underground in the oil shale deposit, is referred to as retorting.
Heat for retorting the oil shale can be obtained by burning some of the carbonaceous material in the shale with air or other oxygen supplying gas. Preferably the oil shale is retorted in situ in a bed of oil shale particles filling a cavity blasted into the oil shale deposit or formation. In such an in situ retort the rubble pile of oil shale particles is ignited at the top to form a combustion zone and air is passed downwardly through the bed to sustain the combustion zone and retort the oil shale on the advancing side of the combustion zone. Liquid oil flows to the bottom of the retort and is recovered.
As gas flows downwardly through the in situ retort three distinct but overlapping zones are created. One of these zones is the combustion zone in which much of the reaction between oxygen supplying gas and carbonaceous material in the oil shale is occurring. This zone may have appreciable thickness since the rate of combustion is to some extent controlled by the rate of diffusion of oxygen supplying gas and reaction products through solid particles of shale.
Above the combustion zone there is a zone of heated spent shale that can contain a substantial amount of unburned residual carbon. This heated shale remains at an elevated temperature long after the combustion zone has passed. Some combustion does occur in this zone of heated spent shale during retorting by reaction between oxygen and residual carbon.
Below the combustion zone in a typical retort the gas is essentially inert since the oxygen has been removed in the hot spent shale and in the combustion zone. This hot substantially oxygen free gas heats the oil shale in a retorting zone, thereby decomposing the kerogen.
These zones progress downwardly through the retort at a rather slow rate of no more than a few feet per day and retorting of a large in situ retort can proceed for a substantial period of time.
To recover the maximum amount of shale oil from a given area a pattern of adjacent retorts is formed. Each of these retorts is filled with a bed of fragmented oil shale particles. Pillars of unfragmented oil shale are left in retorts and between adjacent retorts primarily to act as supports for the overburden of rock above the oil shale deposit being retorted. Typically, for example, each retort is a rectangular room filled with oil shale particles and is bounded on all sides by pillars separating it from adjacent retorts. Even with the best possible mining techniques as much as 30% to 40% of the oil shale may be left in pillars to support the overburden. In recovery schemes where the oil shale is mined and retorted at the surface all of the shale oil in the pillars is sacrificed. In underground in situ retorting some of the oil in the pillars is recovered due to heat transfer from the combustion zone and hot spent shale. This recovery of oil from the pillars is limited by diffusion rates of heat into the pillar and decomposition products out of the pillar, and appreciable amounts of oil may still be left after an area has been completely retorted.
It is therefore desirable to provide a technique for increasing the yield of oil from the pillars of oil shale adjacent in situ oil shale retorts. Such a technique should be sufficiently economical that the cost of the oil is not significantly increased. These techniques should also avoid damage to the structural integrity of the pillars so that mining hazards are not created in the retorting area and ground subsidence of the overburden is avoided. In practice of this invention this is obtained by relatively controlled fracture of essentially intact pillars with faces adjacent the fragmented shale in the in situ retort.
When nuclear devices are detonated underground to create a "chimney" containing rock fragments, there is fracturing of a portion of the rock surrounding the point of detonation of the nuclear device. Detonation pressures plastically and elastically deform surrounding rock, forming a generally spherical cavity with a lining of molten rock. After detonation, the rock above this cavity tends to collapse into the cavity, ultimately creating a vertically elongated chimney containing rock fragments. The deformed rock near this chimney that was subjected to high stresses from the nuclear detonation may contain fractures.
For example, U.S. Pat. No. 3,409,082, by B. G. Bray, et al. for a "Process for Stimulating Petroliferous Subterranean Formations with Contained Nuclear Explosions" states that the pressure waves from the explosion result in fractures in the rocks surrounding and above the detonation location. This complex fracture system results in an increase in the permeability of the matrix rock. Such fracturing is substantially uncontrolled since it is an inherent byproduct of the nuclear detonation. It is believed that most such fracturing is near the point of detonation although minor fractures may extend for a large distance towards the ground surface, as indicated in FIG. 1 of that patent.
Other patents, such as, for example, U.S. Pat. No. 3,698,478 by Harry W. Parker, entitled "Retorting of Nuclear Chimneys", suggest by the drawings that fracturing of the rock around a nuclear chimney is uniform throughout the height. It is believed that this is only a semi-schematic representation and not indicative of an observed effect.